A taut wire fence alarm system utilizes one or more wires stretched across an opening such that the intruder must move or disturb the wire to gain entry into the secured location. Detection circuitry is coupled to the taut wires to detect a deflection of one or more of the wires. The detecting circuitry must be able to distinguish movements in the wires due to an intruder from other movements in the wires, such as movements caused by wind or by small animals. Furthermore, the structure supporting the taut wire must prevent an intruder from overcoming the fence without disturbing the wires.
Taut wire fence alarm systems use anchor posts between which the taut wires are strung. One area of the fence which is particularly difficult to protect from an intruder's climbing is the corner anchor post. One method of preventing an intruder from using the corner anchor post to negotiate the fence is to extend the fence at least five feet past the corner in each direction. In this case, even if an intruder can climb the anchor post, the intruder will still be five feet from the secured area. The disadvantage of this method is that a large amount of real estate may be wasted because of the additional real estate required to extend the fence. Also, this method requires the installation of two posts.
A second method is to use breakaway tabs to secure the wire to the corner anchor post. Using this method, the tabs break under an intruder's weight thereby causing a deflection of the associated wire. This method, however, is only effective when the secure area is outside the corner, i.e., the corner must turn around the threat area.
Another problem associated with prior art taut wire fence alarm system concerns the use of angles within a zone. The fence comprises a plurality of zones, each zone having two anchor posts at the endpoints of the zone and a sensing post located between the anchor posts, typically in the middle. A number of intermediate posts help to support the weight of the wires. The sensing post contains a number of electronic components, and is therefore relatively expensive. Thus, it is desirable to minimize the number of zones.
In the prior art, formation of a non-linear zone, i.e., a zone including one or more angles, was problematic. Angles of greater than fifteen degrees were impossible to achieve at an intermediate post because of the friction occurring between the wires and the loops through which the angle was formed. Friction at angles of greater than fifteen degrees significantly impedes translation movement of the wires. Since translational movement of the wires is important to detection, friction with the wires must be minimized.
Therefore, a need has arisen in the industry for a method and apparatus for providing a fence alarm system with secure anchor posts to efficiently and accurately monitor a secured area.